1. Field of the Invention
The present invention generally relates to a magnetic tape cassette and more particularly, to a front lid locking mechanism of a magnetic tape cassette particularly suitable for perfect dust proof and drip proof purposes.
2. Discussion of Related Art
Generally, in a magnetic recording and reproducing apparatus of a cassette type, the starting end and final end of the magnetic tape are detected by a so-called optical detecting method in which light emitted from a light emitting element inserted into the magnetic tape cassette is detected by light receiving elements disposed at side faces of the tape cassette in a state where the magnetic tape is held between the light emitting element and the light receiving elements. In other words, when transparent leader tapes provided at the winding start portion and the winding end portion of the cassette tape have arrived at the portion between the light emitting element and the light receiving elements, light emitted from the light emitting element transmits through the transparent leader tape, so as to be detected by the light receiving element for determining the starting end or final end of the magnetic tape.
In the above arrangement, on the side wall of the magnetic tape cassette, a tape end detecting window is formed at a predetermined position so as to allow light projected from the light emitting element to pass therethrough.
As one example, in the case of an 8 mm video tape cassette, a square hole of 2.5 mm square is formed at a height of 6.05 mm from a reference face of the tape cassette as specified in the standard. Normally, in the case where a front lid of the tape cassette is closed, the square hole is closed by a side wall of the front lid of the tape cassette, and only when the tape cassette is loaded on a related apparatus and the front lid is opened, will the square hole be exposed to provide a light path for the projecting light.
Incidentally, in the conventional tape cassette as described above, since a locking mechanism of the front lid is disposed in a space between a side wall of the front lid and a side wall of a tape cassette half or tape cassette housing, there is normally formed a gap equivalent to a thickness of a front lid stopper member between the side wall of the tape cassette half and the front lid side wall, in a state when the cassette front lid has been closed. Therefore, in the tape cassette construction as described above, the tape end detecting window is held in a state where it is exposed through the gap, this being disadvantageous not only with respect to dust prevention and drip prevention, but also resulting in deterioration in performance, such as the so-called drop-out effect or the like.
Meanwhile, when the tape end detecting window is provided in a position where it does not overlap the front lid side wall in terms of its plane, it is necessary to separately provide a shutter mechanism for the tape end detecting window, thus complicating the construction of the tape cassette, with consequent lowering in the assembling efficiency and reliability.